K / 1 0 " V B /10~4m K /10"9m 2 B /10"4m DW0007 00 2.47 0.320 4.74 2.65 DW0007 06 2.38 0.185 5.10 2.65 DW0007 07 2.15 0.1 87 5.14 2.67 DW0007 08 2.06 0.175 4.88 1 .87 DW0007 09 1 .36 0.074 4.25 1.10
repeated for this set of foams with the membranes intact. Th e r e s u l t s are g i v e n in t a b l e 3*3 a l o n g w i t h the previously measured values of K and B with the membranes damaged by compression.
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As may be seen from tab le 3*3 the c o ef f i c i e n t s K and B h av e been d r a m a t i c a l l y a l t e r e d by c o m p r e s s i n g the foam p r i o r to t a k i n g the m e a s u r e m e n t s . T y p i c a l l y the p e r m e a b i l i t y has been i n cr ea se d by a factor of 2-3 by rupture of the c e l l membranes. In the case of the coefficient B the values for the foam with the membranes intact are up to 10 times s ma ll e r. Both K and B may be seen to decr ea se s i g n i f i c a n t l y as one m o v e s up the foam series.
The results taken for the B P Chemicals foam series with the membranes intact follow much more closely the trends found by the m an u fac tu rers . The data of table 3*3 i l l u s t r a t e s that the v i b r a t i o n c ontro l a d d i t i v e in use with the foam series changes the a i r - f l o w p ro p e r t i e s by introducing membranes between the cells to restrict fluid flow. This makes the YC additive unsuitable for use with foams in a p p l i c a t i o n s such as cushioning, where large d ef o r m a t i o n s are expected. As the m em b r a n e s ruptured there would be serious degradation of the foams air-flow properties.
The Flow Resistance Coefficient
The f l o w r es is t an c e c o e f f i c i en t ,B ,is a m e a s u r e of the i n e r t i a l f l o w losses g e ne ra t ed by f l o w thro ug h the i r r e g u l a r s t r u c t u r e . It is c h a r a c t e r i s e d by the d i m e n s i o n l e s s c o e f fi c ie n t C^. T a bl e 3.2 g i ve s the v a l u e s of B for a l l the foams considered. The v a l u e of Cf for each foam type was then c a l c u l a t e d from them by means of e qu at io n 3»22, using m e a s ur ed v a l u e s of d from
table 3*1 and assuming that T = f~2~.
E q u a t i o n 3*19 w h i c h is the s t a r t i n g p o i n t for the derivation of the flow resistance coefficient is in fact m o r e p r o p e r l y a d e f i n i t i o n for C^. The f r i c t i o n co e ff i ci e nt w i l l be a f un ct i on of both the r o ug h n e s s of the p assage and the R e y n o l d s number. V a l u e s of Cf from the t es t- pi e ce s c on si d er e d show v e r y l i t t l e s y st e m a t i c d i f fe r en c e b et w ee n c o n v e n t i o n a l and h i g h - r e s i 1 iency samples. In figure 3-9 the f r i c t io n c o e f f i c i e n t is
0.2
1.0
AVERAGE CELL DIAMETER / 2.0* m 3.0
0.5
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